`On 05/22/2011 05:25 PM, George Spelvin wrote:>> This is a little-endian CRC, so x^8+x^7+x^6+x^4+x^2+1 is encoded with>> the x^7 coefficient in bit 0, and the x^0 coefficient in bit 7.  (And the>> x^8 coefficient is implicit and suppressed.)Thanks. However, your code example is confusing.>> You can fill in a CRC table for an arbitrary polynomial with>>>> #define POLY 0xAB	/* 1 + x^2 + x^4 + x^6 + x^7 (+ x^8) */>> typedef uint8_t crc_type;	/* Must be an unsigned type */>>>> void>> crc_le(crc_type const table[256], crc_type crc, u8 const *buf, size_t len)>> {>> 	while (len--)>> 		crc = (crc>>  8) ^ table[(crc ^ *buf++)&  0xff];Here is where my confusion starts. Shifting crc by 8 bits basically means 0 ^ table[], right?>> 	return crc;>> }>>>> If you care, the corresponding code for a big-endian CRC>> (data and CRC transmitted msbit-first) is:>>>> void>> crc_init_be(crc_type table[256], crc_type poly)>> {>> 	int i, j;>> 	crc_type const msbit = ~(~(crc_type)0>>  1);	/* Must be unsigned */>> 	crc_type t = msbit;>>>> 	table[0] = 0;>>>> 	for (i = 1; i<  256; i *= 2) {>> 		t = (t<<  1) ^ (t&  msbit ? poly : 0);>> 		for (j = 0; j<  i; j++)>> 			table[i+j] = table[j] ^ t;>> 	}>> }>>>> void>> crc_be(crc_type const table[256], crc_type crc, u8 const *buf, size_t len)>> {>> 	while (len--)>> 		crc = (crc<<  8) ^ table[(crc>>  (8*sizeof crc - 8)) ^ *buf++];Here is the other shift of crc that does not make sense to me.Gr. AvS-- Almost nobody dances sober, unless they happen to be insane.-- H.P. Lovecraft --`